Department of Chemistry , University of Minnesota , Minneapolis , Minnesota 55455 , United States.
J Phys Chem A. 2019 Sep 19;123(37):7932-7939. doi: 10.1021/acs.jpca.9b02473. Epub 2019 Sep 5.
Femtosecond stimulated Raman spectroscopy (FSRS) is a useful technique for uncovering chemical reaction dynamics by acquiring high-resolution Raman spectra with ultrafast time resolution. However, in FSRS, it can be challenging to discern Raman features from signals arising from transient absorption and other four-wave mixing pathways. To overcome this difficulty, we combine the principles of shifted excitation Raman difference spectroscopy with a simple fixed frequency comb to perform dual-frequency Raman pump FSRS. Through the addition of only two mirrors and a slit to the standard FSRS setup, this method provides Raman spectra at two different excitation wavelengths that can be processed by an automated algorithm to reconstruct the Raman spectrum. Here, we demonstrate the utility of dual-frequency Raman pump FSRS to easily identify Raman signatures by visual inspection for excited-state and ground-state spectra, both on- and off-resonance. We show that previously assigned short-lived vibrations of photoexcited β-carotene are actually not vibrational in nature. We also use crystalline betaine-30 as a challenging test case for this method, as the FSRS spectra contain a number of narrow transient vibronic and non-SRS features. By reliably reducing interference from background signals, the interpretation is substantially more quantitative and enables the future study of new systems, particularly those with small Raman cross-sections or solid-state samples with narrow vibronic features.
飞秒受激拉曼光谱（FSRS）是一种通过超快时间分辨率获取高分辨率拉曼光谱来揭示化学反应动力学的有用技术。然而，在 FSRS 中，从瞬态吸收和其他四波混频途径产生的信号中辨别拉曼特征可能具有挑战性。为了克服这一困难，我们结合移频激发拉曼差谱的原理和简单的固定频率梳，进行双频 Raman 泵 FSRS。通过仅向标准 FSRS 设置添加两个镜子和一个狭缝，该方法提供了在两个不同激发波长下的 Raman 光谱，这些光谱可以通过自动算法进行处理，以重建 Raman 光谱。在这里，我们展示了双频 Raman 泵 FSRS 的实用性，通过对激发态和基态光谱的视觉检查，无论是在共振还是非共振条件下，都可以轻松识别 Raman 特征。我们表明，先前分配给光激发β-胡萝卜素的短寿命振动实际上不是振动性质的。我们还使用结晶甜菜碱-30 作为该方法的挑战性测试案例，因为 FSRS 光谱包含许多窄的瞬态振子和非 SRS 特征。通过可靠地减少背景信号的干扰，解释变得更具定量性，并能够对新系统进行未来的研究，特别是那些具有小 Raman 截面或具有窄振子特征的固态样品的系统。
